A known differential amplifier circuit essentially comprises two input transistors, two load elements, and a current source, which are connected between a fixedly predetermined high and low supply potential. The two input transistors are controlled by an input voltage and by a reference voltage. The current source is generally constructed as a current mirror having an input path and an output path. Differential amplifiers are used in memory components, such as, for example, DDR SDRAMs (double data rate synchronous dynamic random access memory) in order to generate signal states 1 (high) or 0 (low). In this case, an input voltage is compared with a predetermined reference voltage. Depending on whether the input voltage is above or below the reference voltage, the differential amplifier supplies the signal state 1 or 0 at the output. Signal transitions from the state 0 to the state 1 are generated by raising the input voltage from a low voltage value to a high voltage value. In the event of the signal transition from state 1 to state 0, the input voltage of the differential amplifier is lowered from a high voltage level to a low voltage level. As soon as the signal level of the input signal reaches the value of the reference voltage, the signal of the differential amplifier changes from one state to the other.
The value of the reference voltage usually cannot be held at a fixed value, it is subject to fluctuations. Fluctuations of the reference voltage bring about a shift in the operating point of the circuit. This has the effect that the point of intersection at which the level of the input voltage intersects the level of the reference voltage shifts. There is also a further consequence of longer or shorter switching times of the differential amplifier due to an altered switching behavior of input transistors and current source transistor, so that the adaptation or synchronization of a clock signal and of the input signals is impaired.
As a further consequence of these lengthened or shortened switching times of the differential amplifier, there are also influences in the set up and hold times. The set up time is the time period during which a signal must already have assumed a defined state at a data input before the signal is processed further after the occurrence of the clock pulse. The hold time describes the time period during which a signal must still be present at a data output after the clock pulse has been effected. Fluctuations of the reference voltage with the abovementioned consequences mean that the set up and hold times have to be dimensioned such that they are comparatively long.